Hi altp10 let me try to help you out.
Your solution is partially incorrect due to the second part. You are clumping the ‘units’ of students from the hall and field together (you added 1u and 3u to make 4u), and that is the part that went wrong. 1 unit of student from the field and 1 unit of student from the hall are of ‘different size’/different value.
We should distinguish between the units of students from field and hall, respectively.
Hence, following my solution, you can see that instead of naming them ‘units’, I name them ‘F’ and ‘H’. Alternatively we can also name them ‘units’ and ‘parts’. Essentially this is a simultaneous equation that secondary school students solve, but in primary school, we name this concept ‘parts and units‘.
My workings actually pick up from where you made a mistake from. It continues from after the transfer of the students to different locations. Hope it can provide enough information and explanation by itself (:
Hope this solution provided above helps, to anyone reading this as well!
Feel free to PM or text me (: